Referring to FIG. 1, by way of example background, there is illustrated a known conventional fully suspended weigh frame 110 forming part of a belt weighing system 100. Belt weighing system 100 includes idlers (i.e. rollers) 120 spaced apart to support belt 130. Idlers 140 are part of fully suspended weigh frame 110. Conveyed material, for example bulk material or process material used or produced in mining or industrial processing, being transported along belt 130 imparts its weight via belt 130 and idlers 140 and can be measured by weigh frame 110. Accurate weighing is required for bulk handling of materials in many diverse industries, for example mining, ship loading, rail loading, grain, coal power, wool scouring, quarry, food and chemical industries, etc.
Referring to FIG. 2, providing a further example as illustrative background, there is shown a section of a belt weighing system 200. Belt weighing system 200 includes a belt 210 supported by idlers 220 (which should also be read as reference to a set of idlers) that are spaced apart from other belt supporting idlers. Material being transported by belt 210 imparts its weight via belt 210 and idlers 220 and can be measured by a weigh frame of the belt weighing system 200.
There have been many approaches in attempting to develop a reliably accurate conveyor belt weighing system. One particular aspect requiring improvement involves determining a suitably accurate zero measurement and a resultant zero adjustment for actual weight measurements in belt weighing systems.
For many years the zero adjustment of a belt weighing system has involved finding the average weight of the static components of the weighing system plus the dynamic weight of the empty belt. This average zero constant weight is then subtracted from a measured total weight of zero mass and moving material over a weigh frame for every measurement calculation.
However, the resultant integration of total material weighed is only exactly correct once for each complete belt revolution. Recently ‘zero image’ systems have been developed which keep a multiplicity of zero constants in a computer or digital memory for the empty conveyor belt and these systems retrieve the appropriate zero constant for the corresponding piece or section of belt over the weigh frame from the memory when the section of belt is over the weigh zone. With this type of system, the instantaneous weight result is correct within the tolerance of the weighing system at each calculation.
The Applicant has previously improved zero image systems to provide automatic tracking in zero image systems, that is the automatic capture of a correct zero setting for a conveyor belt without operator intervention. This known system provides for the capture of an average zero constant (conventional belt weigher zero) and a new multiplicity of zeroes for the entire belt known as the zero image.
The crux of automatic tracking of a zero adjustment is how to make the decision that the belt is actually empty. Zero shifts of all sizes can occur, for example typical zero shifts might be 0.1%, 0.5% or 2.0% FSD. One known technique to detect an ‘empty belt’ state is to establish a one sided ‘zero window’, being a tolerance above zero that is assumed to be close enough to zero that the belt is probably empty. A typical zero window might be say 4.0% FSD, so a zero error of as much as 2.0% along with natural belt weight variation would likely still fit within the zero window. All weight results less than the zero ‘window’ or ‘zero level’ are deemed to require consideration for calculating a zero adjustment.
In the Applicant's known belt weighing system, a pre-timer is used, so measurements of belt weight are required to be within the zero window, remain there for the duration of time set by the pre-timer and then continue to remain within the zero window for one complete belt revolution. If these requirements are satisfied, then a new average weight for the weigh frame and belt is obtained, which provides a new candidate zero constant. The new candidate zero constant is then exposed to several verification or ‘sanity’ checks before being put into service.
The principle verification check currently used by the Applicant is a ‘zero tracking limit’, being a cumulative amount of zero adjustment that the user is prepared to allow before an alarm is raised to bring the user's attention to the weighing system. This cumulative amount is compared to the last manually initiated Zero Adjustment which becomes the reference for the Limit Alarm. The zero tracking limit might be, for example, 3.0%. This is to say that if the new candidate zero setting now proposed is different by 3.0% or more to the last manually established zero setting then this new candidate zero constant is not allowed, an alarm is raised and the zero is limited to the alarm trigger level. The weigh frame might have been slowly being weighed down with process material and the alarm summons the user to check the weighing system and either clean down the weigh frame or re-zero the weighing system manually, thus re-establishing the reference point for the zero tracking alarm.
Another verification check presently used by the Applicant is the ‘zero skip’ system where the new candidate zero setting is checked against the last ‘x’ zero attempts and when all these agree within y % then the new candidate zero constant may be used. For instance, a new zero constant might be required to be within 0.1% when compared to the previous three zero attempts. If a zero is attempted but does not meet this criteria, the zero constant is stored in memory and a display registers an instance of a ‘zero skip’ having occurred. The system could look say at the last three zero attempts, if all attempts met the criteria then they are used as the last three zero tracks, if there were zero skips in the last three attempts the system looks at the last three zeros anyway, whether successful or not. The zero skip system is a good way of keeping zero constants accurate, but if there is a major zero shift it would take ‘x’, for example three with the example settings, zero attempts to establish a new zero.
In real life, zero adjustment opportunities can be very limited due to continuous operation and multiple zero attempts may never happen. However, improved zero tracking is necessary, in an unattended weighing system zero tracking needs to be foolproof otherwise zero tracking can actually exacerbate zero error problems. For example, attempted automatic tracking of a zero adjustment might easily be ‘tricked’ by a trickle of process material or some other transitory affect.
Another significant disadvantage of known prior art systems is that a conveyor belt must complete a full belt revolution to properly detect a zero condition and propose a new zero adjustment.
There is a need for an improved belt weighing system and/or a method of zero adjustment, tracking or determination which addresses or at least ameliorates one or more problems inherent in the prior art.
The reference in this specification to any prior publication (or information derived from the prior publication), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from the prior publication) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.